THE LOWER VERTEBRATA. 191 



found around the British coasts, S. canicula, the spotted 

 dogfish, being the commonest. The group of sharks to 

 which it belongs is distinguished by the small size of the 

 spiracles, the small pectoral fins and the pattern of the 

 calcification in the vertebral centra, from the group to 

 which some other sharks, and the torpedoes, skates and rays, 

 belong. These two groups together make up the order 

 Elasmobranchii, characterized by the presence of placoid 

 scales and absence of ordinary bones, the separate openings 

 of th~e gill-slits and absence of a gill-cover, the detailed 

 structure of heart and tnmcus arteriosus, the large size and 

 small number of the ova, the absence of a swim-Madder 

 (homologous with lungs), and certain other points, from the 

 much commoner and more familiar fishes, such as cod, 

 salmon and herring, which form a separate order. Much as 

 these two orders differ from one another, they agree in 

 possessing a hyostylic arrangement of the skull ( 12), and 

 so are sharply distinguished from certain rarer groups of 

 fishes, like the mud-fishes of the rivers of the southern 

 hemisphere, which have an autostylic skull, and which in 

 this and other respects are much nearer the land -inhabiting 

 vertebrates. 



